This invention relates to lasers, and more particularly to continuously pumped, repetitively Q-switched lasers employed in resistor trimming and other micromachining operations.
The initial few laser pulses of a string of pulses from a continuously pumped, repetitively Q-switched laser can be much larger than subsequent pulses in the string. This occurs when the laser has not been Q-switching for a time longer than the natural decay time associated with the atomic laser transition and when the Q-switch rate period is of the order of but less than the natural decay rate.
For example, for a Nd:YAG laser, large initial Q-switched pulses occur when the laser output has been quiescent more than about five milliseconds and the repetition rate is above approximately 1 Khz.
Since the Nd:YAG laser is used in thin film resistor trimming and other micromachining on delicate or damage-sensitive substrates, the sequence of laser operation is in the region where it produces damaging large initial pulses. These pulses must be prevented from reaching the sensitive substrate materials.
Traditionally, these initial large pulses are blocked and absorbed by a mechanical shutter which is activated with the laser gating signal and its mechanical time constant provides the required first pulse blocking. However, since the fastest mechanical shutter blocks the beam for a period of several milliseconds, the times involved in the opening and closing movements of the shutter are lost in the trimming or other machining process. For many short machining operations, the fraction of time lost in this manner can represent a significant loss of production.
Further, since the laser is blocked for several pulses by a moving mechanical device, there is presented a problem of synchronizing the beginning of transmitted pulses with the motion mechanism of the trimming or other machining apparatus, since these functions will not begin at the same place for different accelerations or velocity profiles or laser repetition rates.
Additionally, for certain trimming operations where only one or a few laser pulses are required, no reliable method of producing only a finite integer number of pulses is possible with a mechanical shutter.
Still further, the mechanically movable shutter is subject to wear, its operating life being inversely proportional to its operating rate. For high trimming rates of operation, shutter lifetime is excessively short.